Halwaxiida or halwaxiids is a proposed clade equivalent to the older orders Sachitida He 1980 and Thambetolepidea Jell 1981, loosely uniting scale-bearing Cambrian animals, which may lie in the stem group to molluscs or lophotrochozoa. Some palaeontologists question the validity of the Halwaxiida clade.
See the main article Wiwaxia
Wiwaxia is a genus of soft-bodied, scale-covered animals known from Burgess shale type Lagerstätte dating from the upper Lower Cambrian to Middle Cambrian. The organisms are mainly known from dispersed sclerites; articulated specimens, where found, range from 3.4 millimetres (0.13 in) to a little over 50.8 millimeters (2 in) in length. They bear a set of mouthparts comprising two to three identical rows of rasping teeth that lay on a supporting apparatus or "tongue"; this feeding arrangement seems to represent a 'prototype' molluscan radula.
Odontogriphus (literally "toothed riddle") is a genus of soft-bodied animals known from middle Cambrian Lagerstätte. Reaching as much as 12.5 centimetres (4.9 in) in length, Odontogriphus is a flat, oval bilaterian which apparently had a single muscular foot, and a "shell" on its back that was moderately rigid but of a material unsuited to fossilization. Originally it was known from only one specimen, but 189 new finds in the years immediately preceding 2006 made a detailed description possible. (221 specimens of Odontogriphus are known from the Greater Phyllopod bed, where they comprise 0.42% of the community.) As a result Odontogriphus has become prominent in the debate that has gone on since 1990 about the evolutionary origins of molluscs, annelid worms and brachiopods. A group of scientists think that Odontogriphus’s feeding apparatus, which is "nearly identical" to Wiwaxia’s, is an early version of the molluscan radula, a chitinous "tongue" that bears multiple rows of rasping teeth. Hence they classify Odontogriphus and Wiwaxia as close to the ancestors of the first true molluscs. One scientist has presented a different analysis, arguing since 1990 that Wiwaxia is not closely related to molluscs but is much more like a polychaete worm. He argues that the supposed "radula" is nothing of the sort; he classifies Odontogriphus as a basal lophotrochozoan, in other words close to the last common ancestor of molluscs, annelid worms and brachiopods.
Orthrozanclus reburrus ("Dawn scythe with bristling hair") is a sea creature known from the Middle Cambrian (~505 million years ago) Burgess shale, about one centimeter long, with long spikes protruding from its armored body The describers of this fossil animal, Simon Conway Morris and Jean-Bernard Caron, say Orthrozanclus may have formed a link between the halkieriid and the wiwaxiid families, uniting them tentatively in a group called "Halwaxiida", characterized by a similar type of body armor; these organisms might have been stem group molluscs, or fall as a stem group to the larger lophotrochozoan clade (containing molluscs, annelids and brachiopods). However, the status of the Halwaxiid grouping is not universally accepted.
The halkieriids are a group of fossil organisms from the Lower to Middle Cambrian. Their eponymous genus is Halkieria (pron.: /hælˈkɪəriə/), which has been found on almost every continent in Lower to Mid Cambrian deposits, forming a large component of the small shelly fossil assemblages. The best known species is Halkieria evangelista, from the North Greenland Sirius Passet Lagerstätte, in which complete specimens were collected on an expedition in 1989. The fossils were described by Simon Conway Morris and John Peel in a short paper in 1990 in the journal Nature. Later a more thorough description was undertaken in 1995 in the journal Philosophical Transactions of the Royal Society of London and wider evolutionary implications were posed. The group is sometimes equated to Sachitida, although as originally envisaged, this group includes the wiwaxiids and is thus equivalent to the Halwaxiida.
The animals looked like slugs in chain mail - 1.5 centimetres (0.59 in) to 8 centimetres (3.1 in) long, bilaterally symmetric, flattened from top to bottom and unarmored on the bottom. Very near each end there is a shell plate with prominent growth lines rather like the growth rings of trees. The rest of the upper surface was covered with about 2,000 sclerites that overlapped each other like tiles and formed three zones with sclerites of different shapes: "palmates", shaped rather like maple leaves, ran along the center of the back between the shell plates; blade-shaped "cultrates" lay on either side of the palmates and pointing towards the middle of the upper surface; and slim, sickle-shaped "siculates" covered the outer edges. The sclerites bore a wide central cavity, and (at least in some specimens) finer lateral canals. As the animals grew, the shell plates grew by adding material to the outer edges. Individual sclerites stayed the same size; since the cultrate sclerites form a pattern that is constant in all fairly complete specimens, the old ones that were too small may have been shed and replaced by larger ones as the animals grew. The sclerites seem to have grown by basal secretion. There are traces of thin ribs between the sclerites and the skin. The shellplates and the sclerites were probably made of calcium carbonate originally; it has been suggested on the basis of how they were preserved that they may have been wholly organic, but this is less likely since fossils of non-calcified organisms are usually thin films while Halkeieria fossils are three-dimensional like those of trilobites and hyoliths - in fact several specimens show curvature in the horizontal plane, which suggests that the muscles associated with the sclerites were still present at the time of burial The sole was soft and probably muscular. Since Halkieria was unsuited to swimming and had no obvious adaptations for burrowing, it must have lived on the sea-floor, "walking" by making its muscular sole ripple. The backward-projecting siculate sclerites may have improved its grip by preventing it from slipping backwards. Some specimens have been found partially rolled up, rather like a pillbug, and in this position the cultrate sclerites projected outwards, which probably deterred predators. It is difficult to determine the functions of the cap-shaped shells at either end of the animal, as the sclerites appear to have offered adequate protection. Scars on the inner surface of the front shell may indicate that it provided an attachment for internal organs. In one specimen the rear shell appears to have rotated by about 45° before fossilization, which suggests there was a cavity underneath, which may have housed gills. Traces of a gut have been found in the rear halves of some fossils. Parts of one specimen have been interpreted as a radula, the toothed chitinous tongue that is the signature feature of molluscs, but in this specimen the edge of the "scleritome", i.e. coat of sclerites, is folded and the putative radula could be a group of dislocated siculate sclerites.